Itemtype (IN) : Internal name of the item
Itemkey (IN) : A string to uniquely identify the item type instance
Actid (IN) : ID of the activity from where this procedure is called.
Funcmode (IN) : Execution mode of the activity. You can get the activity id from WF_PROCESS_ACTIVITIES table column instance id
Resultout (OUT) : Result Type of the activity

FUNCMODE : For functions, it can be RUN or CANCEL
For notifications, it can be RESPOND, FORWARD, TRANSFER or TIMEOUT

I'm developing a custom form for PO Requisition,the forms shows the open notifications for the user Reviewer.
In that form i'm having 2 button's 'Approve' and 'Reject'. If the Approve button is clicked it will call the PO Requisition Workflow Process Approve_Doc Function.
I have no idea how to call the workflow function inside this form. IF you have any idea or samples help me to do this